High frequency tube apparatus and coupled cavity output circuit therefor



Aprll 1962 R JEPSEN ETAL 3,028,519

HIGH FREQUENCY TUBE APPARATUS AND COUPLED CAVITY OUTPUT CIRCUIT THEREFOR Filed Jan. 2. 1959 2 Sheets-Sheet 1 R-F POWER OUT Output Circuit Driver Section HHMOd 3A LLV'EH INVENTOR.

Robert L. Jepsen BY Richard L. Waher Attorney April 3, 1962 R. L. JEPSEN ETAL 3,028,519

HIGH FREQUENCY TUBE APPARATUS AND COUPLED CAVITY OUTPUT CIRCUIT THEREFOR 2 Sheets-Sheet 2 Filed Jan. 2. 1959 Fig. 4

n r um mp0 a V L L mf 0 h M C RR Y K 4 Murat;

Attorney ite ta tes This invention relates, in general, .to high frequency tube apparatus of the type wherein a current modulated beam of charged particles, such as electrons, is directed through a cavity resonator thereby transforming the energy of the beam into electromagnetic energy; for example, klystron tubes useful in such high frequency applications as navigation and communication systems and high energy linear accelerators. More particularly, the invention relates to high frequency tubes of the above type which may be operated over a wide band of frequencies without requiring mechanical tuning adjustments as, for example, where input signals of varying frequency are applied to the tube in rapid succession.

l-leretofore high frequency broadband systems have generally required tubes of the traveling Wave type. However, in high power applications, that is those requiring tubes capable of handling an instantaneous power as large as several megawatts and an average power as large as several kilowatts, traveling wave tubes tend'to become more narrow in frequency response and give rise to difficult heat dissipation problems due to their small size at high frequencies. Thus a great need has arisen for high power klystron tubes or other cavity resonator tubes which are capable of efficient broadband operation.

It is, accordingly, the object of this invention to provide a high frequency tube apparatus of the type employing a cavity resonator output circuit which is operable without mechanical adjustment over a wide band of frequencies.

One feature of the present invention is the provision of a high power electron beam apparatus, for example, a klystron tube, with a coupled cavity output circuit which enables a heretofore unobtainable combination of high power output over a wide band of frequencies.

Another feature of the present invention is the provision of a high power beam apparatus in accordance with the next preceding paragraph wherein said coupled cavity output circuit comprises an electron beam interaction cavity and a synchronously tuned auxiliary cavity coupled thereto and wherein the loading is concentrated in said auxiliary cavity.

' Still another feature of the present invention is the provision of a high power beam apparatus in accordance with the next preceding paragraph wherein said interacting and auxiliary cavities are formed by a novel sectioned output waveguide structure.

These and other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawing wherein,

FIG. 1 is a schematic representation of a high frequency tube apparatus of the type employing a cavity resonator output circuit,

FIG. 2 is a plot of power v frequency curves for comparing a coupled cavity output circuit in accordance with the present invention with a single cavity output circuit.

FIG. 3 is a plan view, partially broken away, of a novel.

klystron tube in accordance with the present invention,

and

7 FIG. 4 is a detailed cross-sectional view of a portion of the structure in FIG. 3 showing a novel coupled cavity output circuit in accordance with the present invention.

The high frequency tubes to which the present inveninc 3,028,519 Patented Apr. 3, 1962 tion is directed can be resolved into two components distributed along an electron beam as shown schematically in FIG. 1: (1) a driver section whose function is to provide current modulation on the beam, and (2) an ouptut cavity resonator circuit whose function is to extract energy from the modulated beam. In the case of klystron tubes, the driver section comprises an input cavity resonator and a plurality of buncher cavity resonators. However, other types of. driver sections are possible. For example, beam current modulation may be effected by the interaction of the electron beam and an electromagnetic wave propagated on a delay line as is the practice in traveling wave tube design.

In the case of klystron tubes, it is possible to provide a relatively constant current modulation over a wide band of frequencies in the driver section by means of stagger tuning, that is by tuning each of the individual cavity resonators in the driver section to one of a set of frequencies which is distributed over a band which is equal to, or slightly wider than, the desired tube pass-band. Thus, for moderate gains (40 to 50 db), a driver having a practical number of cavities (about 5 or 6) can be stagger tuned so that the overall bandwidth of the tube is limited by the bandwidth of the output circuit.

The half-power bandwidth of a single cavity output circuit, assuming negligible cavity and beam loading losses, may be expressed by the following equation for the inverse of the loaded Q or quality factor of the output cavity:

where:

R is the characteristic impedance of the cavity, and R is the load impedance.

where:

A is the ratio of the peak RF. voltage across the load (which may be built up before the electron turn-around effect sets in) to the beam acceleration voltage,

17 is the DC. to RF. energy conversion efficiency' at band center.

:K is the perveance of the electron beam and P is the peak RF. power output.

Thus a fundamental limitation is placed on the bandwidth of a conventional single output cavity klystron amplifier.

Under typical operating conditions the above parameters have the following approximate values:

K: 1.8 X10 amps/volts 3/2 P=l.4 10 watts R =3600 ohms yielding an optimumly loaded Q=Q of 33 and a halfpower bandwidth of about 3%. The power output of such an optimum'ly loaded single cavity as a function of frequency is shown by the dashed curve in FIG. 2.

In accordance with the present invention it has been determined that the rth-power bandwidth (bandwidth wherein the power always remains greater than a specified fraction, r, of the maximum power available at band center with an optimumly loaded single cavity) can be increased on the order of two times that possible with a single interacting cavity by coupling a second synchronously tuned cavity to the interacting cavity so that the two cavities are tuned to substantially the same frequency near the center of the desired pass-band and concentrating the load in the second cavity, thus requiring only one independent loading adjusting and output coupling for effective power utilization. More particularly, for a range of r values from about 0.5 to 0.8, an optimum rthpower bandwidth improvement over the single cavity by a factor on the order of is obtainable by using a second cavity which is critically coupled to the interacting cavity, or alternatively slightly undercoupled for large r values, and which is loaded such that the quality factor, Q of the coupled cavity is on the order of Q =rQ In accordance with conventional terminology, the coupling between the cavities is defined as passing from an undercoupled value through the critically coupled value to an overcoupled value as the impedance of the output circuit changes from a single-valued to a double-valued function of frequency on a complex impedance plane.

The frequency response for a double cavity output circuit for r= /z is shown by the solid curve in FIG. 2; in this case the half-power bandwidth (r=Vz) is increased by a factor of two over that of the single cavity circuit (dashed curve) having the same interaction gap capacitance when the two cavities are critically coupled and the Q of the coupled cavity is half that of the optimumly loaded single cavity.

Referring to FIG. 3 there is shown a high power multicavity klystron amplifier tube, which may be generally of the type disclosed in the co-pending US. patent application of Richard B. Nelson, et 211., Serial No. 515,327, filed June 14, 1955 and entitled Electron Tube Apparatus, now U.S. Patent 2,944,187, to which reference is made for a more detailed description. Briefly, an electron gun 11 is energized at heater contact 12 to provide a pencil-like beam of electrons longitudinally through the tube. The beam is accelerated through a grounded annular anode member 14 by a high negative potential applied to cathode contact 13, said high negative voltage being insulated from grounded body member 16 by an insulating cylinder 15. The beam then passes through a plurality of cylindrical drift tubes interconnecting a plurality of generally rectangular cavity resonators 21 27 and forming electron interaction gaps 17 therewithin, and is finally terminated in an electron collecting structure 28. The electron beam remains confined by the action of an external magnet structure (not shown) which is mounted between annular magnetic pole pieces 29 and 31. The resonant frequency of the resonators 21-27 is adjusted by a plurality of tuning shafts 32 mounted on pole piece 31 and coupled to tuning assemblies 33 for controlling the position of tuning diaphragms, such as 64 within the individual cavity resonators. The tube is fluid cooled through body fluid fittings 35 and pipes 35 and collector fluid fittings 36. Proper alignment and rigidity of the tube apparatus is maintained by a plurality of stiffener plates 37 and a plurality of stiffener rods 38 passing transversely therethrough. Suitable provision is made for the shielding of x-radiation emitted by the tube as, for example, by a lead shield 39.

Electromagnetic energy which it is desired to amplify is fed into input cavity 21 via coaxial cable 41 and coupling connector 42, then producing a velocity modulation on the electron beam at gap 17 in the first resonator. The beam is further velocity modulated by buncher cavities 22, 23, 24, 2S and 26 thus producing a large current modulation on the beam before it passes through the interaction gap in output cavity 27. The electromagnetic energy induced in cavity 27 by the modulated beam is transmitted therefrom through a sectioned waveguide structure 43 (to be described subsequently) and a wavepermeable vacuum-tight Window 43' at the end thereof and thence to an external waveguide (not shown) mounted on waveguide flange 44.

Referring to FIG. 4 there is shown a more detailed view of the output cavity circuit in accordance with the present invention. The generally rectangularly shaped interacting cavity resonator 27 is closed at one end by a cavity cover 51 secured in a vacuum sealing manner, as by brazing, to cavity wall 52. The opposite end of the cavity 27 is formed by iris plate 53. The cavity 27 is coupled, through iris 53', to an auxiliary cavity resonator 54 which is disposed between the coupling iris plate 53 and an external iris plate 55 forming a discontinuity in section 56 of the rectangular output waveguide structure. The output waveguide is continued in aligned section 57 secured to external iris plate 55 and a curved section 58 secured to section 57. The auxiliary resonator 54 is externally loaded through iris 55'. Leading from waveguide section 57 is an exhaust tube 59, the end of which may be pinched-off and sealed after completion of the exhaustion process. The capacitance of cavity 27 is determined by the design of the interaction gap 17 for maximum utilization of the current modulation on the beam passing therethrough in accordance with conventional practices. The frequency of cavity 27 is inductively tuned by the translation of tuning piston 61 through cavity cover 51, the vacuum of the cavity interior being preserved by a bellows 62 sealed between the piston and the cavity cover. A thin tuning diaphragm 64 as of, for example, copper-plated Monel is secured to piston 61 at diaphragm plate 63 as by riveting and the ends of the diaphragm 64 are inwardly secured, as by brazing, to slits cut on opposite surfaces of wall 52 so that the diaphragm 64 just clears the wall surfaces parallel to the plane of the paper. This wide-range inductive tuner is claimed in copending US. patent application, Serial No. 787,082, filed January 15, 1959 and assigned to applicants assignee.

The frequency of auxiliary cavity 54 is adjusted by varying the spacing between fixed tuning slug 65 and movable tuning slug 66. Translation of tuning slug 66 may be effected by a tuning piston 67 inserted into the cavity interior in the same vacuum tight manner as piston 61. The tuning action of slug 66 may be improved by the inclusion of a thin tuning diaphragm 68 secured between slug 66 and cavity wall 56. A small channel 69 is drilled through slug 66 in order to avoid undesirable pressure differentials on opposite sides of the thin diaphragm 68.

The parameters of the output cavity circuit of FIG. 4 may be conveniently adjusted to obtain the broadband features previously discussed. For example, before final assembly, the iris 55' is first cut into plate 55 until the condition of optimum loading is attained with respect to auxiliary cavity 54. Then the iris 53' is cut into plate 53 until a condition of critical coupling between cavities 27 and 54 is attained. It is to be noted that according to the preferred embodiment of the present invention the auxiliary cavity 54, being formed within the evacuated waveguide structure, may be coupled to the interacting cavity 27 through a relatively thin plate 53 without requiring a vacuum tight window therebetween. This arrangement has the advantage of eliminating serious window cracking and x-radiation problems which arise when a window is placed in the vicinity of a high energy beam; and also undesirable long-line effects where there is a large physical separation of the coupled cavities are eliminated.

An example of a tube constructed in accordance with the present invention is a one megawatt pulsed kylstron amplifier designed for operation in the S-band (1700 to 5000 Inc.) wherein auxiliary cavity 54 was loaded to a value Q /zQ cavities 27 and 54 were tuned to the center frequency of the desired pass-band; the cavities of the driver section were stagger tuned about the center frequency as follows: cavity 21--m.inus 1.8 cavity 22 plus 1.8%; cavity 23plus 3.0%; cavity 24-minus 2.1%; cavity 25plus 2.1%; cavity 26-p1us 2.8%. Under typical operating conditions such a tube was found to have a half-power bandwidth of about as compared with a 2.8% bandwith for a single output cavity tube under similar operating conditions. Substantially greater bandwidth improvement is obtainable at higher power levels and lower frequencies.

Other schemes for coupling an auxiliary cavity to the interacting cavity in accordance with the teachings of the present invention will be apparent to those skilled in the art. For example, the auxiliary cavity need not be re-entrant as is generally the case for the interacting cavity, but may be made of any convenient shape providing a resonant mode near the center (frequency of the desired pass-band.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense..

What is claimed is:

1. A broadband output circuit for a high power, modulated beam, amplifier tube comprising: an unloaded first cavity resonator having an electron beam interaction region therein, a second cavity resonator coupled to said first cavity resonator in the region of critical coupling, said first and second cavity resonators being tuned to substantially the same frequency near the center of the desired pass-band of the tube, and means for coupling said second cavity resonator to an external power utilization load, said second cavity resonator being loaded to a Q value which is about .5 to .8 tifnes the Q value required for maximum power transfer to an output circuit comprising only the first interacting cavity resonator.

2. An output circuit for a high power, modulated beam, amplifier tube wherein the output power over the passband of the tube remains greater than a fraction, r, of the maximum power available at band center with an optimumly loaded single cavity output circuit comprising: an unloaded first cavity resonator having an electron beam interaction region therein, a second cavity resonator tuned to substantially the same frequency as said first cavity resonator near the center of the desired pass-band and coupled thereto in a region of critical coupling, and means for coupling said second resonator to an external power utilization load, said second cavity resonator being loaded to a Q value which is r times the Q value required for maximum power transfer when only the first interacting cavity resonator is loaded.

3. In a broadband klystron amplifier tube, the combination comprising: a driver section for producing a current modulation on an electron beam passable therethrough, said driver section including a plurality of cavity resonators which are centrally tuned to a set of different frequencies distributed over a band of frequencies'which at least partially includes the desired pass-band of the tube; and an output circuit for efficiently extracting power from said current modulated beam over said passband, said output circuit comprising an unloaded first cavity resonator, a second cavity resonator coupled to said first cavity resonator, said first and second cavity resonators being tuned to frequencies within said passband, and means for coupling said second resonator to an external power utilization load.

4. In a broadband k'lystron amplifier tube, the combination comprising: a driver section for producing a cur-- rent modulation on an electron beam passable therethrough, said driver section including a plurality of cavity resonators which are tuned to a set of frequencies distributed over a band of frequencies which at least partially includes the desired pass-band of the tube; and an output circuit for efiiciently extracting power from said current modulated beam over said pass-band, said output circuit comprising an unloaded first cavity resonator having an electron interaction region therein, an evacuated portion of a waveguide structure communicating with said first cavity resonator, means for electromagnetically coupling said waveguide portion and said first cavity resonator, and means forming a discontinuity in said waveguide structure for providing a second cavity resonator between said coupling means and said discontinuity means.

5. The combination of claim 4 wherein said coupling means comprises a first iris structure scoured transversely within said waveguide.

6. The combination of claim 5 wherein said discontinunity forming means comprises a second iris structure secured transversely within said waveguide.

7. The combination of claim 6 wherein said first cavity resonator is provided with inductive wall tuning means and said second cavity is provided with re-entrant capacitive tuning means.

8. The'combination of claim 4 further including a wave-permeable vacuum-tight window positioned between said Waveguide portion and an external utilization load which said waveguide structure is adapted to receive.

References Cited in the file of this patent UNITED STATES PATENTS Re. 22,990 Hansen et al. Mar. 23, 1948 2,425,738 Hansen Aug. 19, 1947 2,517,731 Sproull Aug. 8, 1950 2,591,910 Barford Apr. 8, 1952 2,606,302 Learned Aug. 5, 1952 2,610,307 Hansen et a1 Sept. 9, 1952 2,657,314 Kleen et al Oct. 27, 1953 2,658,147 Bainbridge Nov. 3, 1953 2,790,928 Reed Apr. 30, 1957 FOREIGN PATENTS 921,166 Germany Dec. 9, 1954 1,143,187 France Apr. 8, 1957 804,463 Great Britain Nov. 19, 1958 

